It depends on the context. For instance:
1. If Bill Clinton were a footballer, he would probably play wide.
2. If Bill Clinton was a footballer, it's surprising that Wikipedia doesn't mention the fact.
The if-clause in #1 presents a speculation; the "were" is subjunctive. The if-clause in #2 presents historical information, however; the "was" is indicative.
It is also common to hear or see "was" in place of "were", in sentences of the #1 kind.
Not a professional ESL teacher.